Short takeoff and landing vehicle with forward swept wings

ABSTRACT

A vehicle includes a tilt rotor that is aft of a wing and that is attached to the wing via a pylon. The tilt rotor has an adjustable maximum downward angle from horizontal that is less than or equal to 60° and that is set via a setting associated with a flight computer. The vehicle takes off and lands using at least some lift from the wing and from the tilt rotor. In response to a change to the adjustable maximum downward angle, via the setting associated with the flight computer, which produces a new maximum downward angle: the flight computer updates an actuator authority database associated with the flight computer to reflect the new maximum downward angle. Using the updated actuator authority database that reflects the new maximum downward angle, the flight computer generates a rotor control signal for the tilt rotor.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/066,058 entitled SHORT TAKEOFF AND LANDING VEHICLE WITH FORWARD SWEPTWINGS filed Oct. 8, 2020 which is incorporated herein by reference forall purposes, which claims priority to U.S. Provisional PatentApplication No. 62/912,872 entitled FIXED WING AIRCRAFT WITH TILT ROTORSfiled Oct. 9, 2019 which is incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION

New types of aircraft are being developed which are capable of takingoff and landing in dense urban areas, opening up new transportationpathways and bypassing gridlock on the roads. For example, Kitty HawkCorporation is developing a new electric vertical takeoff and landing(eVTOL) tiltrotor which can take off and land in a footprint of roughly30 ft.×30 ft. An early prototype has been manufactured and tested andfurther improvements to the vehicle's performance (e.g., improving therange) would be desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1A is a perspective view diagram illustrating an embodiment of aforward swept, fixed wing vehicle with tilt rotors.

FIG. 1B is a top view diagram illustrating an embodiment of a forwardswept, fixed wing vehicle with tilt rotors.

FIG. 2A is a diagram illustrating a bottom view of an embodiment ofboundary layer thicknesses with the motors off.

FIG. 2B is a diagram illustrating a bottom view of an embodiment ofboundary layer thicknesses with motors on.

FIG. 3A is a diagram illustrating an example of a tilt wingconfiguration with corresponding lift vector, thrust vector, and drag.

FIG. 3B is a diagram illustrating an example of a fixed wingconfiguration with a leading edge mounted tilt rotor and correspondinglift vector, thrust vector, and drag.

FIG. 3C is a diagram illustrating an embodiment of a fixed wingconfiguration with a trailing edge mounted tilt rotor and correspondinglift vector, thrust vector, and drag.

FIG. 4 is a diagram illustrating an embodiment of airflow produced whentrailing edge mounted tilt rotors on a main wing are off.

FIG. 5 is a diagram illustrating an embodiment of a forward swept andtapered wing and a straight wing for comparison.

FIG. 6A is a diagram illustrating an embodiment of a takeoff tilt changefrom hover position to cruise position.

FIG. 6B is a diagram illustrating an embodiment of a landing tilt changefrom cruise position to hover position.

FIG. 7 is a diagram illustrating an embodiment of a velocity tiltdiagram.

FIG. 8 is a diagram illustrating an embodiment of a vehicle with atruncated fuselage which is capable of flying in a magic carpet mode.

FIG. 9A is a top view diagram illustrating an embodiment of a vehiclewith a truncated fuselage and tail.

FIG. 9B is a side view diagram illustrating an embodiment of a vehiclewith a truncated fuselage and tail.

FIG. 10A is a side view diagram illustrating an embodiment of a STOLvehicle with rotors in a 30° position from horizontal.

FIG. 10B is a perspective view diagram illustrating an embodiment of aSTOL vehicle with rotors in a 30° position from horizontal.

FIG. 11 is a diagram illustrating a side view of a STOL vehicle withrotors in a 60° position from horizontal.

FIG. 12 is a diagram illustrating a side view of a STOL vehicle withrotors in a 60° position from horizontal.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

Various embodiments of a short takeoff and landing (STOL) vehicle aredescribed herein. In some embodiments, vehicle includes a tail having asurface and a fuselage having a surface, where the tail and the fuselagehave a continuity of surfaces where the surface of the tail directlycoupled to the surface of the fuselage. This embodiment of the vehiclefurther includes a forward-swept wing having a trailing edge and a rotorthat is attached to the trailing edge of the forward-swept wing via apylon, where the rotor has a maximum downward angle from horizontal thatis less than or equal to 60° and the STOL vehicle takes off and landsusing at least some lift from the forward-swept wing and at least somelift from the rotor.

It may be helpful to first describe an early prototype of the tiltrotorvehicle, where the tilt rotors have a range of (substantially) 0°-90°.Then, various embodiments of the STOL vehicle are described.

FIG. 1A is a perspective view diagram illustrating an embodiment of aforward swept, fixed wing vehicle with tilt rotors. FIG. 1B is a topview diagram illustrating an embodiment of a forward swept, fixed wingvehicle with tilt rotors. In the example shown, the main wing (100 a and100 b) is a fixed wing which is attached to the fuselage (102 a and 102b) in a fixed manner or position. The main wing is not, in other words,a tilt wing which is capable of rotating. The main wing (100 a and 100b) is also forward swept (e.g., relative to the pitch axis). Forexample, the forward-sweep angle may be on the order of ° sweep between14° and 16° for aircraft embodiments with a canard (as shown here) or ashigh as 35° for aircraft embodiments without a canard.

In this example, the main wing (100 a and 100 b) has six rotors (104 aand 104 b) which are attached to the trailing edge of the main wing.Rotors or propellers in this configuration are sometimes referred to aspusher propellers (e.g., because the propellers are behind the wing and“push” the vehicle forward, at least when they are in their forwardflight position). Forward flight mode is sometimes referred to herein ascruise mode. For clarity, these rotors on the main wing are sometimesreferred to as the main wing rotors (e.g., to differentiate them fromthe rotors which are attached to the canard). Naturally, the number ofrotors shown here is merely exemplary and is not intended to belimiting.

In addition to the six main wing rotors, there are two rotors (106 a and106 b) which are attached to the canard (108 a and 108 b). These rotorsare sometimes referred to as the canard rotors. The canard is thinnerthan the main wing, so unlike the main wing rotors, the canard rotorsare attached to the distal ends of the canard as opposed to the trailingedge of the canard.

All of the rotors in this example are tilt rotors, meaning that they arecapable of tilting or otherwise rotating between two positions. In thedrawings shown here, the rotors are in a cruise (e.g., forward flight,backward facing, etc.) position. In this position, the rotors arerotating about the (e.g., substantially) longitudinal axes of rotationso that they provide (substantially) backward thrust. When the rotorsare in this position, the lift to keep the tiltrotor vehicle airbornecomes from the airflow over the main wing (100 a and 100 b) and thecanard (108 a and 108 b). In this particular example, the rotationalrange of a tilt rotor ranges has a minimum angular position ofapproximately 0°-5° and a maximum angular position of approximately90°-95°. This range is design and/or implementation specific.

The rotors can also be tilted down to be in a hover (e.g., verticaltakeoff and landing, downward facing, etc.) position (not shown). Inthis second position, the rotors are rotating about (e.g.,substantially) vertical axes of rotation so that they provide(substantially) downward thrust. In this configuration, the lift to keepthe tiltrotor vehicle airborne comes from the downward airflow of therotors.

Generally speaking, the tilt rotors, when oriented to output thrustsubstantially downward, permit the aircraft to perform vertical takeoffand landings (VTOL). This mode or configuration (e.g., with respect tothe manner in which the aircraft as a whole is flown and/or with respectto the position of the tilt rotors specifically) is sometimes referredto as hovering. The ability to perform vertical takeoffs and landingspermits the aircraft to take off and land in areas where there are noairports and/or runways. Once airborne, the tilt rotors (if desired)change position to output thrust (substantially) backwards instead ofdownwards. This permits the aircraft to fly in a manner that is moreefficient for forward flight; this mode or configuration is sometimesreferred to as cruising.

A canard is useful because it can stall first (e.g., before the mainwing), creating a lot of pitching moments and not much loss of lift atstall whereas a main wing stall loses a lot of lift per change inpitching moment (e.g., causing the entire aircraft to drop or fall).Stalls are thus potentially more benign with a canard compared towithout a canard. The canard stall behavior is particularly beneficialin combination with a swept forward wing, as the stall of the main wingcan create an adverse pitching moment if at the wing root and can createlarge and dangerous rolling moments if at the wing tip. Furthermore, acanard can create lift at low airspeeds and increase CL_(max) (i.e.,maximum lift coefficient) and provides a strut to hold or otherwiseattach the canard motors to.

In some embodiments, the pylons (110 a and 110 b) which are used toattach the rotors to the canard and/or main wing include some hingeand/or rotating mechanism so that the tilt rotors can rotate between thetwo positions shown. Any appropriate hinge mechanism may be used. Forexample, with ultralight aircraft, there are very stringent weightrequirements and so a lightweight solution may be desirable.Alternatively, a fixed-tilt solution may also be used to meet verystringent weight requirements.

In some embodiments, the aircraft is designed so that the main wing (100a and 100 b) and canard (108 a and 108 b) are able to provide sufficientlift to perform a glider-like landing if needed during an emergency. Forexample, some ultralight standards or specifications require the abilityto land safely if one or more rotors fail and the ability to perform aglider-like landing would satisfy that requirement. One benefit to usinga fixed wing for the main wing (e.g., as opposed to a tilt wing) is thatthere is no danger of the wing being stuck in the wrong position (e.g.,a hover position) where a glider-like landing is not possible because ofthe wing position which is unsuitable for a glider-like landing.

Another benefit to a fixed wing with trailing edge mounted tilt rotorsis stall behavior (or lack thereof) during a transition from hoverposition to cruise position or vice versa. With a tilt wing, duringtransition, the tilt wing's angle of attack changes which makes stallingan increased risk. A fixed wing with trailing edge mounted tilt rotorsdoes not change the wing angle of attack (e.g., even if rotors areturned off/on or the tilt rotors are shifted). Also, this configurationboth adds dynamic pressure and circulation over the main wing, whichsubstantially improves the behavior during a transition (e.g., fromhover position to cruise position or vice versa). In other words, thetransition can be performed faster and/or more efficiently with a fixedwing with trailing edge mounted tilt rotors compared to a tilt wing (asan example).

Another benefit associated with fixed wing vehicle with tilt rotors(e.g., as opposed to a tilt wing) is that a smaller mass fraction isused for the tilt actuator(s). That is, multiple actuators for multipletilt rotors (still) comprise a smaller mass fraction than a single,heavy actuator for a tilt wing. There are also fewer points of failurewith tilt rotors since there are multiple actuators as opposed to asingle (and heavy) actuator for the tilt wing. Another benefit is that afixed wing makes the transition (e.g., between a cruising mode orposition and a hovering mode or position) more stable and/or fastercompared to a tilt wing design.

In some embodiments, the rotors are variable pitch propellers which havedifferent blade pitches when the rotors are in the hovering positionversus the cruising position. For example, different (ranges of) bladepitches may enable more efficient operation or flight when in the cruiseposition versus the hovering position. When the rotors are in a cruiseposition, putting the blade pitches into “cruising pitch” (e.g., on theorder of 26°) enables low frontal area which is good for cruising (e.g.,lower drag). When the rotors are in a hovering position, putting theblade pitches into a “hovering pitch” (e.g., on the order of 6°) enableshigh disc area which is good for hovering. To put it another way, oneblade pitch may be well suited for cruising mode but not for hoveringmode and vice versa. The use of variable pitch propellers enables better(e.g., overall) efficiency, resulting in less power consumption and/orincreased flight range.

The following figures illustrate various benefits associated with theexemplary aircraft shown in FIGS. 1A and 1B.

FIG. 2A is a diagram illustrating a bottom view of an embodiment ofboundary layer thicknesses with the motors off. In this example, laminarrun lines 200 a, 202 a, and 204 a illustrate laminar runs at variousregions of the main wing. In this example, it is assumed that theaircraft is cruising (e.g., flying in a substantially forwarddirection). As in FIGS. 1A and 1B, the main wing rotors (206) areattached to the trailing edge of the main wing (208) in this embodiment.The next figure shows the boundary layer thicknesses with the rotorsturned on.

FIG. 2B is a diagram illustrating a bottom view of an embodiment ofboundary layer thicknesses with motors on. In this example, the motorsare on and the rotors have an exit airflow velocity of 30 m/s. With themotors on, a low pressure region is created towards the aft of the wingwhich increases the laminar run on the main wing. See, for example,laminar run lines 200 b, 202 b, and 204 b which correspond to laminarrun lines 200 a, 202 a, and 204 a from FIG. 2A. A comparison of the twosets illustrates that the laminar runs have increased for the first twolocations (i.e., at 200 a/200 b and 202 a/202 b). The last location(i.e., 204 a/204 b) has only a slightly longer laminar run length due tointerference from the canard rotors (210).

The drag from the main wing rotors (more specifically, the drag from thepylons which are used to attach the main wing rotors to the main wing)is hidden in the wake of the airflow coming off the main wing. See, forexample FIG. 2A which more clearly shows that the pylons (220) areconnected or otherwise attached behind most of the extent of laminar run(222). With the embodiment shown here, the pylons also get to keep someof the boundary layer thickness from the main wing, which means thepylons have lower drag per surface area. This improves the drag comparedto some other alternate designs or configurations. The following figuresdescribe this in more detail.

FIG. 3A is a diagram illustrating an example of a tilt wingconfiguration with corresponding lift vector, thrust vector, and drag.In this example, a fixed rotor (300) is attached to a tilt wing (302) ata fixed position or angle. This is one alternate arrangement to theaircraft embodiment(s) described above. To direct the airflow producedby the fixed rotor (300) either backwards or downwards, the tilt wing(302) is rotated. As shown here, with this configuration, there is drag(304) at the trailing edge of the tilt wing, which is undesirable.

The lift (306) and thrust (308) for this configuration are also shownhere, where the tilt wing is shown in the middle of a transition (e.g.,between a cruising position and a hovering position). As shown here, thelift (306) and thrust (308) are substantially orthogonal to each other,which is inefficient. In other words, a tilt wing is inefficient duringits transition.

FIG. 3B is a diagram illustrating an example of a fixed wingconfiguration with a leading edge mounted tilt rotor and correspondinglift vector, thrust vector, and drag. In this example, a tilt rotor(320) is attached to the leading edge of a fixed wing (322). This isanother alternate arrangement to the aircraft embodiment(s) describedabove. The corresponding drag (324) and thrust (326) for thisarrangement are also shown. There is no useful lift produced with thisconfiguration and therefore no lift vector is shown here.

FIG. 3C is a diagram illustrating an embodiment of a fixed wingconfiguration with a trailing edge mounted tilt rotor and correspondinglift vector, thrust vector, and drag. In this example, the tilt rotor(340) is attached to the trailing edge of the fixed wing (342). In thisconfiguration, the drag due to the trailing edge mounted tilt rotor(e.g., mostly due to its pylon, not shown) is hidden in the wake of theairflow coming off the main wing. As such, there is no drag (at leastdue to the tilt rotor (340)).

The position of the trailing edge mounted tilt rotor (340) relative tothe fixed wing (342) also sucks air (344) over the fixed wing, afterwhich the air turns or bends through the rotor and downwards. This flowturning over the wing generates a relatively large induced lift (346)which is shown here. The thrust vector (348) due to the rotors is alsoshown here. It is noted that the induced lift (346) and thrust (348) aresubstantially in the same direction (i.e., both are pointingsubstantially upwards) which is a more efficient arrangement, includingduring a transition. In other words, using a fixed wing with trailingedge mounted tilt rotors produces less drag and improved efficiencyduring a transition (e.g., due to the lift and thrust vectors which nowpoint in substantially the same direction) compared to other rotor andwing arrangements. Note for example, drag 304 and drag 324 in FIG. 3Aand FIG. 3B, respectively, and the orthogonal positions of lift 306 andthrust 308 in FIG. 3A.

The following figure illustrates an embodiment of flow turning in moredetail.

FIG. 4 is a diagram illustrating an embodiment of airflow produced whentrailing edge mounted tilt rotors on a main wing are off. In thisexample, a tiltrotor (400) is shown but with the main wing rotors turnedoff for comparison purposes. With the rotors off, the airflow in (402)and the airflow out (404) are moving in substantially the samedirection. That is, the airflow does not turn (e.g., downwards) as itpasses through the rotors.

Tiltrotor 420 shows the same vehicle as tiltrotor 400 except the rotorsare turned on. In this example, the airflow in (422) and the airflow out(424) have noticeable different directions and there is noticeableturning or bending of the airflow as it passes through the rotors of theexemplary tiltrotor shown. As described above, this induces a noticeablelift, which is desirable because less power is consumed and/or the rangeof the tiltrotor increases.

In this example, the main wing rotors (426) are in the hoveringposition. As shown here, these rotors are slightly pitched or otherwiseangled (e.g., with the tops of the main wing rotors pointing slightlyforward and the bottoms pointing slightly backward). In this diagram,the amount of tilting is shown as θ_(pitch) (428) and in someembodiments is on the order of 90° of rotational range or movement(e.g., ˜3° up from horizontal when in a cruise position (e.g., forminimum drag) and ˜93° degrees down from horizontal when in a hoverposition which produces a rotational range of ˜96°). Although thisangling or pitching of the rotors is not absolutely necessary for flowturning to occur, in some embodiments the main wing rotors are angled orotherwise pitched to some degree in order to increase or otherwiseoptimize the amount of flow turning. In some embodiments, the canardrotors are similarly pitched. It is noted that tiltrotor 420 is shown ina nose up position and therefore the vertical axis (e.g., relative tothe tiltrotor) is not perpendicular to the ground and/or frame ofreference.

In some embodiments, the rotors (e.g., the main wing rotors and/orcanard rotors) are rolled or otherwise angled slightly outward, awayfrom the fuselage, when the rotors are in hovering position. In someembodiments, this roll (e.g., outward) is on the order of 10° forgreater yaw authority.

In some embodiments, the main wing is tapered (e.g., the wing narrowsgoing outward towards the tip) in addition to being forward swept. Thefollowing figures describe various wing and/or tail embodiments.

FIG. 5 is a diagram illustrating an embodiment of a forward swept andtapered wing and a straight wing for comparison. In the example shown,wing 500 is a straight wing with no tapering (e.g., the wing is the samewidth from the center to the tip of the wing). Exemplary rotors (502)are shown at the trailing edge of the straight wing (500).

The center of thrust (504), indicated by a dashed and dotted line, isdictated by the placement or arrangement of the rotors and runs throughthe centers of the main wing rotors (502). For simplicity, the canardrotors are ignored in this example. The center of lift is based on theshape of the wing. For a rectangular wing such as wing 500, the centerof lift (506), indicated by a solid line, runs down the center of thewing. Calculation of the aerodynamic center is more complicated (e.g.,the aerodynamic center depends upon the cross section of the wing, etc.)and aerodynamic center 508, indicated by a dashed line, is exemplaryand/or typical for this type of wing.

As shown here, the straight wing (500) and its corresponding arrangementof main wing rotors (502) produce a center of thrust (504) which isrelatively far from both the center of lift (506) as well as theaerodynamic center. This separation is undesirable. More specifically,when the main wing rotors (502) are in hover position, if the center ofthrust (504) is far from the center of lift (506), then the transition(e.g., in the context of the movement of the aircraft as a whole, suchas switching from flying substantially upwards to substantially forwardsor vice versa) would create very large moments and could overturn thevehicle or prevent acceleration or stability and/or require a massiveand/or non-optimal propulsion system. In cruise, if the center of thrust(504) is far from the center of lift (506), it is not as important(e.g., since the thrust moments are both smaller and more easilybalanced by aerodynamic moments), but it is still undesirable.

In contrast, the forward swept and tapered wing (520) and itscorresponding arrangement of rotors (522) along the trailing edgeproduce a center of thrust (524), center of lift (526), and aerodynamiccenter (528) which are closer to each other. For example, the forwardsweep of the wing brings the rotors forward to varying degrees. Thiscauses the center of thrust to move forward (e.g., towards the leadingedge and towards the other centers). The tapering of the wings preventsthe aerodynamic center and center of lift from creeping forward too much(and more importantly, away from the center of thrust) as a result ofthe forward sweep. For example, with a forward swept wing with notapering (not shown), the center of thrust would move forwardapproximately the same amount as the aerodynamic center and center oflift and would result in more separation between the three centers thanis shown here with wing 520.

Some other benefits to a forward swept and tapered wing include betterpilot visibility, and a better fuselage junction location with the mainwing (e.g., so that the main wing spar can pass behind the pilot seat,not through the pilot). Furthermore, the taper reduces wing moments andputs the center of the thrust of the motors closer to the wingattachment to the fuselage, as referenced about the direction of flight,so there are less moments carried from wing to fuselage, a shorter tailboom (e.g., which reduces the weight of the aircraft), and improvedpitch stability.

The following figures describe exemplary tilt transitions of the rotorsbetween cruise position and hover position.

FIG. 6A is a diagram illustrating an embodiment of a takeoff tilt changefrom hover position to cruise position. In some embodiments, theexemplary tiltrotor performs this transition soon after taking off(e.g., substantially vertically). It is noted that this tilt transitionis optional and the aircraft may fly entirely with the rotors in thehovering position (albeit with less than optimal performance). Forexample, this could be done if there is risk in the tilting action, andit would be better to take the action at a higher altitude.

Tiltrotor 600 shows the exemplary aircraft after it has performed avertical takeoff. In this state shown here, the main wing rotors andcanard rotors are in hover position (e.g., rotating about asubstantially vertical axis of rotation so that the rotors generatesubstantially downward thrust).

The tiltrotor then transitions from an entirely upward direction ofmovement to a direction of movement with at least some forward motionwith the rotors remaining in the hover position until the tiltrotorreaches some desired altitude at which to begin the transition (602). Inother words, the vehicle transitions first, and then changes the tilt ofthe rotors. In one example, the altitude at which the tiltrotor beginsthe rotor tilt change from hover position to cruise position is analtitude which is sufficiently high enough for there to be recovery timein case something goes wrong during the transition. Switching the rotorsbetween hover position and cruise position is a riskier time where thelikelihood of something going wrong (e.g., a rotor failing, a rotorgetting stuck, etc.) is higher. Although the tiltrotor may have systemsand/or techniques in place for recovery (e.g., compensating for a rotorbeing out by having the remaining rotors output more thrust, deploy aparachute, etc.), these systems and/or techniques take time (i.e.,sufficient altitude) to work.

From position 602, the tiltrotor flies substantially forward and movesthe tilt rotors from a hover position (e.g., where thrust is outputsubstantially downward) to a cruise position. Once in the cruiseposition 604, the rotors rotate about a substantially longitudinal axisso that they output backward thrust.

FIG. 6B is a diagram illustrating an embodiment of a landing tilt changefrom cruise position to hover position. For example, the exemplarytiltrotor may perform this transition before landing vertically. As withthe previous transition, this transition is optional. For example, theexemplary tiltrotor can keep the tilt rotors in cruise position andperform a glider-like landing as opposed to a vertical landing ifdesired.

Tiltrotor 610 shows the rotors in a cruise position. While flying in asubstantially forward direction, the tilt rotors are moved from thecruise position shown at 610 to the hover position shown at 612. Withthe tilt rotors in the hover position (612), the tiltrotor descends withsome forward movement (at least in this example) so as to keep power uselow(er) and retain better options in the case of a failure of a motor orother component (e.g., the tiltrotor can power up the rotors and pullout of the landing process or path) to position 614 until it finallylands on the ground.

FIG. 7 is a diagram illustrating an embodiment of a velocity tiltdiagram. In the diagram shown, the x-axis shows the forward speed of theaircraft and the y-axis shows the tilt (e.g., position or angle of thetilt wing or tilt rotors) which ranges from a (e.g., minimal) cruiseposition (700) to a (e.g., maximal) hover position (702).

The first operating envelope (704), shown with a solid border and filledwith a grid pattern, is associated with a tilt wing aircraft. See, forexample, tiltrotor 400 in FIG. 4 and tilt wing 302 and fixed rotor 300in FIG. 3A. The second operating envelope (706), shown with a dashedborder and gray fill, is associated with an (e.g., comparable) aircraftwith a forward swept and fixed wing with trailing edge mounted tiltrotors. See, for example, the embodiments described above.

In the diagram shown here, the tilt rotor operating envelope (706) is asuperset of the tilt wing operating envelope (704) which indicates thatthe former aircraft configuration is safer and/or more airworthy thanthe latter and is also able to fly both faster and slower at comparabletilt positions. With a fixed wing, the wing is already (and/or always)pointed in the direction of (forward) travel. When the tilt rotors areat or near the (e.g., maximal) hover position (702), the vehicle can flypretty much all the way up to the stall speed (e.g., V₂) without havingto tilt the motors up to cruise position. Note, for example, that thetilt rotor operating envelope (706) can stay at the (e.g., maximal)hover position (702) all the way up to V₂. This greatly increases theoperating regime of the tilt rotor operating envelope (706) compared tothe tilt wing operating envelope (704). Note for example, all of thegray area above the tilt wing operating envelope (704).

Another effect which can contribute to the expanded operating envelopefor the tilt rotor configuration at or near hover position includes flowturning (see, e.g., FIG. 4 ). The flow turning over the main winginduces some extra lift. In some embodiments, this flow turning and itsresulting lift are amplified or optimized by tilting the main wingrotors at a slight backward angle from directly down when in a normalhover (e.g., at minimal tilt position 700).

In contrast, a tilt wing presents a large frontal area when the tiltwing is tilted up in (e.g., maximal) hover position (702). As a result,tilt wings are unable to fly forward at any kind of decent speed untilat or near the full (e.g., minimal) cruise position (700) or nearly so.

The example vehicle described above is designed for vertical takeoffsand landing. However, taking off and landing vertically (i.e., hovermode) is very power hungry mode of a flight. By modifying the VTOLvehicle to instead perform short takeoffs and landings (STOL), the rangeof the vehicle may be increased. To adapt the vehicle for STOLoperation, the range of the tilt rotors is restricted to angle(s)meaningfully less than the 90° associated with hovering. The followingfigures describe various embodiments of this.

FIG. 8 is a diagram illustrating an embodiment of a vehicle with atruncated fuselage which is capable of flying in a magic carpet mode. Asused herein, the term magic carpet mode refers to a mode in which therotors are still in a hovering orientation, but the vehicle has beenaccelerated to an airspeed where a substantial amount of lift isgenerated by the wing. In the magic carpet mode, the vehicle speed canbe controlled with forward pitch, and altitude can be controlled eitherby increasing speed to gain efficiency and thus climb rate, or bydirectly adding thrust to the rotors. In the example shown, the vehiclehas a canard (800) with two canard rotors (802), only one of which isshown. The main wing (804), which is a fixed wing with a forward sweep,has six main wing rotors (806), only half of which are shown, which areattached to the trailing edge of the main wing. The fuselage (808) isrelatively short and is referred to herein as a truncated fuselage. Forexample, note that the end of the fuselage (810) extends only a littlebit past the end of the backmost rotor (812). In this particularembodiment, the rotors are fixed and do not tilt or otherwise changeposition.

There are a variety of vehicle embodiments which are capable of meetingstringent weight requirements (e.g., an ultralight standard). In thisapproach, the truncated fuselage is much shorter and there is no tailper se, both of which keep the weight down. The use of fixed rotors(e.g., as opposed to tilt rotors) also keeps the weight down. Thetruncated fuselage and lack of a tail also produces a smaller footprintwhich helps with transport (e.g., in a trailer) and the amount of spacerequired for takeoff and/or landing.

In some embodiments, the rotors at a fixed position tilted back, more onthe hover end of the tilt spectrum as opposed to the cruise end of thetilt spectrum (e.g., an axis of rotation that is tilted downward fromhorizontal at an angle between 20° to 40°, inclusive). See, for examplethe axis of rotation (820) associated with fixed rotor (822) where thetilt angle is between 20° to 40° which suitable and/or acceptable formagic carpet mode. For example, this rotor position (although fixed)permits the exemplary vehicle to fly vertically (e.g., not due toaerodynamic lift on the wing, but from the airflow produced by therotors) as well as forwards (e.g., off the wing). This ability or modeof keeping the rotors in a hover-style tilt while flying (e.g.,primarily and/or mostly) in a wing borne manner is sometimes referred toas a fly magic carpet mode. It is noted that this ability to fly in amagic carpet mode is not necessarily limited to fixed rotor embodiments.For example, some or all of the above tilt rotor embodiments may beflown in magic carpet mode (e.g., where the tilt position is the extremeor maximal hover position, or some tilt position between the twoextremes).

FIG. 9A is a top view diagram illustrating an embodiment of a vehiclewith a truncated fuselage and tail. The embodiment shown here hassimilarities with the previous vehicle embodiment shown in FIG. 8 andfor brevity shared features are not discussed herein. Unlike theprevious example, this embodiment has a tail (900). The fuselage (902 a)is a truncated fuselage so the tail (900) and fuselage (902) areconnected using a boom (904 a).

FIG. 9B is a side view diagram illustrating an embodiment of a vehiclewith a truncated fuselage and tail. FIG. 9B continues the example ofFIG. 9A. From this view, the truncated fuselage (902 b) and the boom(904 b) are shown, as well as other features of the vehicle, including ahorizontal control surface (906) and a vertical control surface (908) onthe tail and ski-like landing gear (910) are more clearly shown.

In some applications, the truncated fuselage vehicles shown in FIGS.8-9B may be undesirable (for reasons described in more detail below).The following figures show various embodiments of a STOL vehicle whichmay be more desirable in such applications.

FIG. 10A is a side view diagram illustrating an embodiment of a STOLvehicle with rotors in a 30° position from horizontal. FIG. 10B is aperspective view diagram illustrating an embodiment of a STOL vehiclewith rotors in a 30° position from horizontal. In the example shownhere, the canard rotors (1000 a and 1000 b) and the main wing rotors(1002 a and 1002 b) are in a 30° position from the horizontal, cruiseposition.

In some embodiments, the rotors (1000 a, 1000 b, 1002 a, and 1002 b) donot tilt and are fixed in the position shown here (i.e., 30°).Alternatively, in some other embodiments, the rotors are able to tiltbetween a horizontal, cruise position (i.e., 0°) and the position shownhere (i.e., 30°). In either case, the vehicle is not be configured forvertical takeoffs and landings but instead uses a (short) runway forshort take off and landings (e.g., 200 ft.). In some applications, thereis space for a short runway and the (cruise) range extension offered bythe STOL vehicle is attractive. In some applications, there is runwayspace but not 200 ft. of runway space. Using a tilt angle that is lowerthan 30° reduces the runway length (but will also affect the range ofthe vehicle and the maximum velocity).

A benefit to the rotors being in the position shown in FIGS. 10A-10B isthat it increases the dynamic pressure over the main wing (1004) fromthe main wing rotors (1002 a and 1002 b), producing higher liftcoefficients (e.g., compared to a comparable VTOL vehicle such as thatshown in FIGS. 1A and 1B). In other words, takeoff and landings for theSTOL vehicle are more efficient compared to a comparable VTOL vehicle attakeoff and landing so less power is needed from the drivetrain. Interms of thrust-to-weight ratio, the exemplary STOL vehicles needs abouthalf of what a comparable VTOL vehicle needs to take off or land.

Another benefit is that because of the reduced power demands on thepowertrain during takeoff and landing, a smaller and lighter powertrainmay be used for the exemplary STOL vehicle compared to a comparable VTOLvehicle. This weight saving may be used to carry more payload or morebatteries (e.g., to extend the range while keeping the total weightsubstantially the same). For example, if extra batteries are added tothe STOL vehicle, a cruise range increase of ˜25% may be realized.

Additional weight savings may be realized if the rotors or propellersare fixed. With fixed rotors the tilt mechanism can be removed. If thisadditional weight saving is used for (still) more batteries, the rangemay increase ˜30%-35% over the comparable VTOL vehicle.

In some embodiments, the rotors are downsized (e.g., smaller blades aswell as smaller motors and power electronics for the motors) relative toa comparable VTOL vehicle (e.g., FIGS. 1A and 1B) due to the reducedpower demands associated with short takeoffs and landings compared tovertical takeoffs and landings. This may be attractive because inaddition to weight savings, better packaging can be achieved due to thesmaller components which reduces drag and also reduces the space neededfor hangaring (e.g., since the overall vehicle width is narrower).

In some embodiments, the (range of) blade pitch angle(s) for a STOLrotor is adjusted (e.g., compared to a comparable VTOL vehicle) due tothe more restrictive (range of) pitch angle(s). For example, with afixed rotor, an optimal blade pitch angle for that fixed tilt positionmay be selected. Or, if the rotor is a tilt rotor, the range of bladepitch angles may be reduced and/or adjusted as a result of the reducerange of motion tilt positions (e.g., with a maximum tilt position inthe range of 20°-40°, or 30°-60°).

Unlike the truncated fuselage vehicles shown in FIGS. 8-9B, the STOLvehicle shown in FIGS. 10A-10B has a full length fuselage (1008 a and1008 b) that is directly coupled to the tail (1006 a and 1006 b), asopposed to a truncated fuselage that is coupled to the tail via a boom.Another way of describing this is that a surface of the tail (directly)meets or contacts a surface of the fuselage so that there is no surfacediscontinuity (e.g., due to a boom). This surface continuity may bedesirable for aerodynamic reasons. As a result of the differentconfigurations, there may be different ranges of tilt angles that areappropriate for the different configurations, as well different methodsof flying, performance characteristics, and/or associated benefits. Insome applications, the STOL vehicle shown in FIGS. 10A-10B is moreattractive than the truncated fuselage vehicles shown in FIGS. 8-9B. Forexample, due to the passive stability of the exemplary STOL vehicle, itmay more attractive for use in manned applications since the controlsystem need not be certified to the same level of redundancy and/orrobustness. In the event of a failure of the system, the STOL vehicle isstill capable of being controlled by a pilot without an automaticcontrol system during cruise the vehicle can be landed via aconventional landing.

The following figures show STOL vehicles with rotors at various tiltpositions and/or angles and runway lengths, ranges, and maximumvelocities associated with those embodiments.

FIG. 11 is a diagram illustrating a side view of a STOL vehicle withrotors in a 60° position from horizontal. In this example, the canardrotors (1100) and the main wing rotors (1102) are in a 60° position fromthe horizontal, cruise position. In various embodiments, the rotors aretilt rotors or fixed rotors.

FIG. 12 is a diagram illustrating a side view of a STOL vehicle withrotors in a 60° position from horizontal. In this example, the canardrotors (1200) and the main wing rotors (1202) are in a 45° position fromthe horizontal, cruise position. As described above, in variousembodiments the rotors are tilt rotors or fixed rotors.

TABLE 1 Example metrics associated with different (maximum) tilt anglescorresponding to FIGS. 10A-12. The range values assume a weight savingsin the STOL’s drivetrain which permit the STOL vehicle to carry morebatteries compared to a comparable VTOL vehicle (Max) Tilt Angle RunwayLength Range* Max Velocity 30° Longer runway than Longer range thanSlower max velocity than VTOL VTOL VTOL Longer runway than 45° Longerrange than 45° Slower max velocity than (max) tilt angle (max) tiltangle 45° (max) tilt angle 45° Longer runway than Longer range thanSlower max velocity than VTOL VTOL VTOL Shorter runway than 30° Shorterrange than 30° Faster max velocity than (max) tilt angle (max) tiltangle 30° (max) tilt angle

Table 1 illustrates various (e.g., performance) metrics and/orcharacteristics of the different (maximum) tilt angles shown in FIGS.10A-12 . Naturally, depending upon the application and/or performancemetrics of interest, different (maximum) tilt angles are more attractivethan others.

In some embodiments, a common or shared code base for the flightcomputer is used for the various embodiments described above, includingfixed rotors and tilt rotors. For example, for safety and/orcertification reasons, it may be desirable to maintain a singlecollection of code for the flight computer. In one example, there is a(e.g., global) parameter or setting associated with the tilt range. Forexample, for a fixed rotor at 30° the range is [30°, 30° ] but for atilt rotor with a maximum tilt angle of 60° the range is [0°, 60° ](e.g., where 0° is the rotor in the cruise (i.e., forward flight)position). This global parameter is propagated throughout and is used bythe flight computer (code) to determine (as an example) the rotor thrustcontrol signals to achieve the desired forces and moments given thepermitted range of tilt angles. For example, the permitted range wouldfirst be used to limit the actuator authority database and then used todetermine which (flight) modes (e.g., hover or STOL takeoff andlanding), are permitted. The controller can then use the subset of modesavailable and actuator authority maps or database to determine how bestto allocate usage of actuators throughout the flight (e.g., duringthrust allocation which generates motor control signals for the motorcontrollers). Using the same code base may also make it easier tosupport a fleet of configurable vehicles that can be reconfigured asdesired (described in more detail below).

In some embodiments, it may be desirable to have a fleet of configurablevehicles that can be reconfigured as desired (e.g., depending uponcurrent demand and/or flight applications). In some embodiments, thispermits a vehicle to be configured for VTOL (e.g., per FIGS. 1A and 1B)or STOL (e.g., per FIGS. 10A-12 ) as desired. In some embodiments, tochange the (maximum) tilt angle, the rotor may include physical devicesthat limit the (maximum) tilt angle, such as hard stops of differentsizes. In some embodiments, the STOL vs. VTOL configuration is donepurely via software limits (e.g., setting a parameter associated with arange of permitted tilt angles).

In some embodiments, the vehicle includes modular and/or swappable partsso that parts that are better suited for VTOL vs. STOL or fixed rotorvs. tilt rotor can be supported. For example, the brackets supportingthe tilt mechanism and motor may not be bonded to the carbon structurebut bolted to a support frame which in turn is bonded to the structure.This would allow a tilt hinge mounted rotor to be swapped out for afixed angle rotor relatively quickly. While the weight associated withsuch a setup would be greater than configurations which cannot bechanged easily, the ability to use a common fleet might offset this insome cases.

Various embodiments of STOL vehicles have been described herein butthese examples are merely exemplary and are not intended to be limiting.In various embodiments, a vehicle includes a tandem wing or a singlewing arrangement designed for STOL operations that may be used for longrange applications with vehicle size ranging from a single personvehicle size to sizes comparable to regional aircraft. In some suchembodiments, the tilt would be restricted to 45°-60° from horizontal(i.e., the forward flight configuration). Although runways would beneeded to land, they could be drastically shortened (e.g., as short as100-200 feet). This permits flights to or from smaller airports or evenlarger heliports that have adequate available space. In someapplications, such vehicles are used for (e.g., dedicated) flights toand from entities (e.g., universities or businesses) having adequatelylarge campuses. By restricting the tilt to (as an example) 45°-60°creates a flap effect allowing the main wing to generate higher liftwithout stalling due to induced flow from the propellers.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A vehicle, comprising: a tilt rotor that is aftof a wing and that is attached to the wing via a pylon, wherein: thetilt rotor has an adjustable maximum downward angle from horizontal thatis less than or equal to 60°; the adjustable maximum downward angle isset via a setting associated with a flight computer; and the vehicletakes off and lands using at least an amount of lift from the wing andat least an amount of lift from the tilt rotor; and the flight computerconfigured to instruct the tilt rotor to produce a new maximum downwardangle in response to a change to the adjustable maximum downward anglevia the setting associated with the flight computer, including by:updating an actuator authority database associated with the flightcomputer to reflect the new maximum downward angle; and generating arotor control signal for the tilt rotor using the updated actuatorauthority database that reflects the new maximum downward angle.
 2. Thevehicle recited in claim 1, wherein the wing includes a forward-sweptand tapered wing.
 3. The vehicle recited in claim 1, wherein the tiltrotor includes a plurality of hard stops corresponding to differentmaximum downward angles.
 4. The vehicle recited in claim 1, wherein thetilt rotor is capable of rotating between a cruise position and theadjustable maximum downward angle.
 5. The vehicle recited in claim 1,wherein the new maximum downward angle is determined based at least inpart on a length of a runway.
 6. The vehicle recited in claim 1, whereinthe new maximum downward angle is determined based at least in part on adesired range for the vehicle.
 7. The vehicle recited in claim 1,wherein in response to the change to the adjustable maximum downwardangle, the flight computer further updates a set of permitted flightmodes associated with the flight computer to reflect the new maximumdownward angle.
 8. The vehicle recited in claim 1, wherein: the newmaximum downward angle is at 90°; and in response to the change to theadjustable maximum downward angle, the flight computer updates a set ofpermitted flight modes associated with the flight computer to include ahover flight mode.
 9. A method, comprising: providing a tilt rotor thatis aft of a wing and that is attached to the wing via a pylon, wherein:the tilt rotor is included in a vehicle; the tilt rotor has anadjustable maximum downward angle from horizontal that is less than orequal to 60°; the adjustable maximum downward angle is set via a settingassociated with a flight computer; and the vehicle takes off and landsusing at least an amount of lift from the wing and at least an amount oflift from the tilt rotor; and providing the flight computer, wherein theflight computer is configured to instruct the tilt rotor to produce anew maximum downward angle in response to a change to the adjustablemaximum downward angle via the setting associated with the flightcomputer, including by: updating an actuator authority databaseassociated with the flight computer to reflect the new maximum downwardangle; and generating a rotor control signal for the tilt rotor usingthe updated actuator authority database that reflects the new maximumdownward angle.
 10. The method recited in claim 9, wherein the wingincludes a forward-swept and tapered wing.
 11. The method recited inclaim 9, wherein the tilt rotor includes a plurality of hard stopscorresponding to different maximum downward angles.
 12. The methodrecited in claim 9, wherein the tilt rotor is capable of rotatingbetween a cruise position and the adjustable maximum downward angle. 13.The method recited in claim 9, wherein the new maximum downward angle isdetermined based at least in part on a length of a runway.
 14. Themethod recited in claim 9, wherein the new maximum downward angle isdetermined based at least in part on a desired range for the vehicle.15. The method recited in claim 9, wherein in response to the change tothe adjustable maximum downward angle, the flight computer furtherupdates a set of permitted flight modes associated with the flightcomputer to reflect the new maximum downward angle.
 16. The methodrecited in claim 9, wherein: the new maximum downward angle is at 90°;and in response to the change to the adjustable maximum downward angle,the flight computer updates a set of permitted flight modes associatedwith the flight computer to include a hover flight mode.